Hi Martin,

How are you? We had this below email conversation in March about the longitudinal template and it’s stability. You had mentioned that with 5 time points, my template should be stable enough to patch another timepoint. I have another study that has only 2 time points and I was reading through your 2012 Longitudinal analysis paper that had used 2 time points to compare with cross-sectional data. I was wondering if you had any suggested literature for the number of time points needed for stability of a template.

Thanks

Koushik Govindarajan

From: Martin Reuter [mailto:mreuter@nmr.mgh.harvard.edu]
Sent: Friday, March 14, 2014 9:31 AM
To: Govindarajan, Koushik Athreya; freesurfer@nmr.mgh.harvard.edu
Subject: Re: [Freesurfer] adding time points to longitudinal base template

Hi Koushik,

there is a way to add time points without recreating the base:
recon-all -long <tpNsubjid> <longbasesubjid> -addtp -all

This is what the recon-all help says about that:
"If a new timepoint needs to be added to a longitudinal run where a base subject
has already been created (from prior timepoints), then the -addtp command
can be added to the -long command in order to 'fix-up' the longitudinal
stream to accept the new timepoint. Note that the base subject is *not*
recomputed using the new timepoint. This potentially introduces a bias, and it
is recommended to NOT add a time point this way! Instead recreate the base
from all time points and run all longitudinals again."

So it's up to you. But given you already have 5 time points in your base and add only 1, you may be fine. The base should be pretty stable with 5 and if the 6th is not too different, it should not matter much. I don't think anyone has ever done a thorough analysis about adding time points.

Best, Martin

On 03/11/2014 10:08 AM, Govindarajan, Koushik Athreya wrote:

Dear FS experts,

I have a serial study in which I have run cross-sectional and longitudinal analysis on 5 time points. Now, I want to add a 6^th time point to my dataset. Do I have to rerun my base template creation step from scratch or is there a way to just add further time points to an existing template?

Thanks

Koushik
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Martin Reuter, Ph.D.

Assistant in Neuroscience - Massachusetts General Hospital

Instructor in Neurology   - Harvard Medical School

MGH / HMS / MIT

A.A.Martinos Center for Biomedical Imaging

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